A bending press has a pair of bending beams and a bending roller engageable between these beams. The workpiece is laid with its one side on the bending beams and the bending roller is pressed in a bending direction against the other side of the workpiece between the beams. The pressure is such as to exceed the elastic limit of the workpiece so that the workpiece is plastically and permanently deformed.
It is necessary during such a bending process to determine the extent of this permanent deformation. Normally this is done simply by measuring the position of the bending roller relative to the position of the bending beam. Such a system has the considerable disadvantage that no compensation is made for the residual elastic deformation of the workpiece. This elastic deformation must be allowed for by multiplying the measurement obtained with a factor depending on workpiece material, workpiece dimensions, workpiece temperature, and other similar factors. Even with such compensation it is very difficult to obtain an exact measurement of the radius of curvature of the bend imparted to the workpiece. A more accurate arrangement requires the removal of the workpiece from the bending press so that the residual elastic deformation imparted thereto can be relieved and the permanent or plastic deformation directly measured, usually using a template.
In a bending operation wherein, for instance, steel plates are bent into cylindrically arcuate shapes it is necessary to monitor the radius of curvature imparted to the workpieces with each step of the bending operation. This is necessary because the bending characteristics change from workpiece to workpiece and it is essential that the finished cylindrically arcuate sections all be of identical overall diameter so as to fit together to form a storage tank or the like. In such an arrangement the necessity of frequently removing the workpiece from the bending press and measuring the radius of curvature of the workpiece is disadvantageous, as it slows production considerably.